Lens system having diffracting surface and refracting surface and optical apparatus using the lens system

ABSTRACT

In a lens system having one or a plurality of diffracting surfaces and one or a plurality of refracting surfaces, a power of each surface and a distance between the surfaces are set such that an optical characteristic of the lens system, such as a focal length (focal point) and/or a spherical aberration, remains substantially unvarying relative to temperature variations within a predetermined range and that an achromatic effect is substantially attained for the focal length (focal point) and/or the spherical aberration within a predetermined region of wavelength.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a lens system having adiffracting surface and a refracting surface, and more particularly to alens system advantageously adapted for the image forming optical systemof an optical apparatus such as an interchangeable lens, a camera, acopying apparatus, a semiconductor-device-manufacturing exposure device,a microscope, a binocular, a telescope, an optical recording/reproducingapparatus, a projector, or the like.

[0003] 2. Description of Related Art:

[0004] In a lens system in which a material having a high thermalexpansion coefficient, such as a plastic material, is used for a lens,variations in temperature cause a change in refractive index of thematerial and also cause the material to expand or contract to change theshape of the lens. The optical characteristics of the lens system wouldbe affected by the temperature variations if such changes are leftuncorrected. Therefore, it is necessary to have some correcting means.To meet this requirement, it is usual to prestore in a memory or thelike the amounts of changes in optical characteristics (such as focus,etc.) anticipated to be caused by temperature variations, have atemperature sensor arranged in the vicinity of the lens, and makenecessary correction by reading an applicable amount of change of theoptical characteristics due to the temperature variations from thememory in accordance with the output of the temperature sensor(corresponding to the amount of change of temperature or to thetemperature). However, this correcting method necessitates the use ofadditional means such as the temperature sensor and the memory, therebycausing an increase in cost of the lens system. To solve that problem, amethod of arranging the lens system itself to be capable of correctingchanges of optical characteristics due to temperature variations withoutusing such correcting means has been developed, for example, asdisclosed in U.S. Pat. No. 5,260,828. According to U.S. Pat. No.5,260,828, a diffracting optical system (diffracting surface) such as akinoform or the like and a refracting optical system (refractingsurface) such as an ordinary lens are combined with each other in such away as to have temperature characteristics opposite to each other.

[0005] In the lens system which is formed by combining the diffractingoptical system and the refracting optical system, the color dispersioncharacteristic of the refracting optical system is inverse to that ofthe diffracting optical system. Therefore, it is known that anachromatic effect (achromatism) can be attained by arranging the powerof the diffracting optical system (1/“focal length”) to be of the samesign as that of the power of the refracting optical system (1/“focallength”).

[0006] However, according to the specification of the above-mentionedU.S. Pat. No. 5,260,828, a relation obtained between the diffractingoptical system and the refracting optical system in correctingvariations of optical characteristics caused by variations intemperature is as follows. 1) The power of the diffracting opticalsystem is at least 20% in absolute values. 2) In a plastic-used lenssystem, the power of the diffracting optical system is either largerthan the power of the refracting optical system or of opposite sign.

[0007] Further, according to the specification of the above-mentionedU.S. Pat. No. 5,260,828, the relation between the diffracting opticalsystem and the refracting optical system to be arranged for achromatism,i.e., in attaining an achromatic effect, is as follows. The power of thediffracting optical system is less than 15% in absolute values, and thepower of the diffracting optical system is smaller than the power of therefracting optical system and of the same sign.

[0008] As mentioned above, the lens system formed by combining thediffracting optical system and the refracting optical system, asdisclosed in U.S. Pat. No. 5,260,828, is arranged to do nothing forachromatism while correcting the variations of optical characteristicscaused by temperature variations. In that lens system, therefore, achromatic aberration inevitably takes place, thereby lowering theperformance of the lens system.

BRIEF SUMMARY OF THE INVENTION

[0009] It is an object of the invention to provide a lens system havinga high rate of performance and an optical apparatus using the lenssystem.

[0010] In accordance with an aspect of the invention, there is provideda lens system having one or a plurality of diffracting surfaces and oneor a plurality of refracting surfaces, in which a power of each surfaceand a distance between the surfaces are set such that an opticalcharacteristic of the lens system, such as a focal length (focal point)and/or a spherical aberration, remains substantially unvarying relativeto temperature variations within a predetermined range and that anachromatic effect is substantially attained for the opticalcharacteristic (the focal length (focal point) and/or the sphericalaberration) within a predetermined region of wavelength.

[0011] Further, in accordance with another aspect of the invention,there is provided a lens system having one or a plurality of diffractingsurfaces and one or a plurality of refracting surfaces, in which a powerof each surface and a distance between the surfaces are set such that anoptical characteristic of the lens system, such as a focal length (focalpoint) and/or an amount of spherical aberration, remains the same at twodifferent temperatures and that the optical characteristic (the focallength (focal point) and/or the amount of spherical aberration) remainsthe same at two different wavelengths.

[0012] Further, in accordance with a further aspect of the invention,there is provided an optical apparatus using one of the above-statedlens systems as an image forming optical system, in which the opticalapparatus is an interchangeable lens, a camera, a copying apparatus, asemiconductor-device-manufacturing exposure device, a microscope, abinocular, a telescope, an optical recording/reproducing apparatus, aprojector or the like.

[0013] The above and other objects and features of the invention willbecome apparent from the following detailed description of embodimentsthereof taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0014]FIG. 1 shows a lens system arranged according to the invention asan embodiment thereof.

[0015]FIG. 2 shows by way of example how the power of the lens systemshown in FIG. 1 varies in relation to variations of wavelength.

[0016]FIG. 3 shows by way of example how the power of the lens systemshown in FIG. 1 varies in relation to variations of temperature.

[0017]FIG. 4 shows a lens system arranged according to the invention asanother embodiment thereof.

[0018]FIG. 5 shows by way of example how the power of the lens systemshown in FIG. 4 varies in relation to variations of wavelength.

[0019]FIG. 6 shows by way of example how the power of the lens systemshown in FIG. 4 varies in relation to variations of temperature.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, preferred embodiments of the invention will bedescribed in detail with reference to the drawings.

[0021]FIG. 1 shows a first embodiment of the invention. In the firstembodiment shown in FIG. 1, a lens system is composed of a single lens,for the sake of simplification of illustration. However, the lens systemof course may be changed to include two or more lenses, instead of justone. In FIG. 1, reference numeral 1 denotes a hybrid lens systemcomposed of a diffracting optical system and a refracting opticalsystem. A light flux incident on the lens system 1 is bent to a certaindegree of angle at a first surface 2 having a diffracting surface, by adiffracting action of the diffracting surface and also by a refractingaction of the curvature of a convex surface of the base of the lenssystem 1. Then, after passing through the material of the base of thelens system 1, the light flux is bent at a second surface 3 having adiffracting surface, by a diffracting action of the diffracting surfaceand a refracting action of the curvature of another convex surface ofthe base of the lens system 1. The light flux is thus caused to convergeon a focal plane 4. In FIG. 1, the diffracting surface is illustratedexaggeratedly in a deformed shape in an optical axis direction. Inactuality, letting a wavelength as a datum be denoted by λ₀ and arefractive index obtained at the wavelength λ₀ of the material whichforms the diffracting surface (diffraction grating) be denoted by n₀,the depth h in the optical axis direction of the diffraction grating canbe expressed as follows:$h = {\frac{m\quad \lambda_{0}}{\left( {n_{0} - 1} \right)}\quad {\left( {m\quad {being}\quad a\quad {positive}\quad {number}} \right).}}$

[0022] If the value m is assumed to be 1, the datum wavelength λ₀ to be540 nm and the refractive index n₀ to be 1.49409, the depth h is 1.092μm. Incidentally, a diffraction grating which forms the diffractingsurface of each of the first surface 2 and the second surface 3 of thelens system 1 has a finite power (1/“focal length”).

[0023] Referring to FIG. 1, the power of the diffracting surface of thefirst surface 2 is assumed to be φ_(a) and the curvature of the base atthe first surface 2 is assumed to be r₁. Then, assuming that the lightentrance side of the first surface 2 is air, a refractive index N ofwhich is 1, and the base is a material whose refractive index N is n, apower φ₁ of the refracting optical system of the first surface 2 can beexpressed as: φ₁=(n−1)/r₁. Since, in the first surface 2, thediffracting surface and the refracting surface exist on a common surfacewith a distance between them at “0” in this instance, a composite powerφ_(1a) of the first surface 2 can be expressed as φ_(1a)=φ_(a)+φ₁. Inthe similar way, the power of the diffracting surface of the secondsurface 3 is assumed to be φ_(b) and the curvature of the base at thesecond surface 3 is assumed to be r₂. Then, assuming that the lightexits into air of the refractive index N=1 from the material of therefractive index N=n, a power φ₂ of the refracting optical system of thesecond surface 3 can be expressed as φ₂=(1−n)/r₂. Then, since, in thesecond surface 3, the diffracting surface and the refracting surfaceexist on a common surface with a distance between them at “0”, acomposite power φ_(2b) of the second surface 3 can be expressed asφ_(2b)=φ_(b)+φ₂. A distance between the first surface 2 and the secondsurface 3 is assumed to be “d” and to be filled with the material of therefractive index “n” as mentioned above.

[0024] Here, the concrete construction of the lens system 1 will bedescribed by way of example. The base of the lens system 1 is made of aPMMA (polymethyl methacrylate), which is a kind of plastic material, toperform an achromatic action with respect to wavelengths 540 nm and 610nm and to have no variation take place in the power of the entire lenssystem as an optical characteristic within a range of temperaturevariation amount of 20 degrees.

[0025] The characteristics of the PMMA used for the base of the lenssystem 1 are first described as follows. The refractive indices no and nof the PMMA obtained respectively at wavelengths 540 nm and 610 nm are1.49409 and 1.49078 (n₀=1.49409 and n₁=1.49078). Further, thecoefficient of linear expansion a of the PMMA is α=6.74×10⁻⁵, and therefractive index variation $\frac{n}{t}$

[0026] of the PMMA due to temperature variations is expressed below:$\frac{n}{t} = {{- 11.5} \times {10^{- 5}.}}$

[0027] Here, the power φ_(T) of the entire lens system 1 can beexpressed as follows: $\begin{matrix}\begin{matrix}{\varphi_{T} = {\varphi_{1\quad a} + \varphi_{2\quad b} - {\frac{d}{n}{\varphi_{1\quad a} \cdot \varphi_{2\quad b}}}}} \\{= {\left( {\varphi_{a} + \varphi_{1}} \right) + \left( {\varphi_{b} + \varphi_{2}} \right) - {\frac{d}{n}{\left( {\varphi_{a} + \varphi_{1}} \right) \cdot {\left( {\varphi_{b} + \varphi_{2}} \right).}}}}}\end{matrix} & (1)\end{matrix}$

[0028] Assuming that the powers of elements obtained at the wavelengthvalue λ₀=540 nm are expressed by the variables mentioned above, they canbe normalized to have the power φ_(T) of the entire lens system at “1”expressed by the following formula: $\begin{matrix}\begin{matrix}{\varphi_{T} = {\left( {\varphi_{\quad a} + \varphi_{\quad b}} \right) + \left( {\varphi_{\quad 1} + \varphi_{\quad 2}} \right) - {\frac{d}{1.49409}\left( {{\varphi_{a}\varphi_{\quad b}} + {\varphi_{1}\varphi_{\quad 2}} + {\varphi_{a}\varphi_{\quad 2}} + {\varphi_{b}\varphi_{\quad 1}}} \right)}}} \\{= 1.}\end{matrix} & (2)\end{matrix}$

[0029] Next, assuming that the power of the entire lens system 1 at thewavelength λ₁=610 nm is expressed as φ_(T)′, the power φ_(T)′ of theentire lens system 1 is obtained as follows. The diffraction gratingmentioned above has a feature which is represented by the followingrelation: ${\frac{\lambda}{\varphi} = {constant}},$

[0030] wherein λ represents wavelength, and φ represents power.

[0031] Therefore, the power φ_(a)′ of the diffracting surface of thefirst surface 2 obtained at the wavelength λ₁ can be expressed asfollows:$\varphi_{a}^{\prime} = {\frac{\lambda_{1}}{\lambda_{0}}{\varphi_{a}.}}$

[0032] The power φ_(b)′ of the diffracting surface of the second surface3 likewise can be expressed as follows:$\varphi_{b}^{\prime} = {\frac{\lambda_{1}}{\lambda_{0}}{\varphi_{b}.}}$

[0033] The power of the refracting optical system, on the other hand,becomes as follows:

[0034] The power φ₁′ of the refracting optical system obtained on thefirst surface 2 is expressed as follows:$\varphi_{1}^{\prime} = {\frac{n_{1} - 1}{r_{1}} = {\frac{n_{1} - 1}{n_{0} - 1}{\varphi_{1}.}}}$

[0035] On the second surface 3, the power φ₂′ of the refracting opticalsystem becomes as expressed below:$\varphi_{2}^{\prime} = {\frac{1 - n_{1}}{r_{2}} = {\frac{n_{1} - 1}{n_{0} - 1}{\varphi_{2}.}}}$

[0036] Therefore, the power φ_(T)′ of the entire lens system 1 can beexpressed as follows: $\begin{matrix}{\varphi_{T}^{\prime} = {{\left( {\varphi_{a}^{\prime} + \varphi_{b}^{\prime}} \right) + \left( {\varphi_{1}^{\prime} + \varphi_{2}^{\prime}} \right) - {\frac{d}{n_{1}}\left( {{\varphi_{a}^{\prime}\varphi_{b}^{\prime}} + {\varphi_{1}^{\prime}\varphi_{2}^{\prime}} + {\varphi_{a}^{\prime}\varphi_{2}^{\prime}} + {\varphi_{b}^{\prime}\varphi_{1}^{\prime}}} \right)}} = \quad {{\frac{\lambda_{1}}{\lambda_{0}}\left( {\varphi_{a} + \varphi_{b}} \right)} + {\frac{n_{1} - 1}{n_{0} - 1}\left( {\varphi_{1} + \varphi_{2}} \right)} - {\frac{d}{n_{1}}{\left\{ {{\frac{\lambda_{1}^{2}}{\lambda_{0}^{2}}\varphi_{a}\varphi_{b}} + {\frac{\left( {n_{1} - 1} \right)^{2}}{\left( {n_{0} - 1} \right)^{2}}\varphi_{1}\varphi_{2}} + {{\frac{\lambda_{1}}{\lambda_{0}} \cdot \frac{n_{1} - 1}{n_{0} - 1}}\left( {{\varphi_{a}\varphi_{2}} + {\varphi_{b}\varphi_{1}}} \right)}} \right\}.}}}}} & (3)\end{matrix}$

[0037] In order to attain the achromatic effect for the wavelengths 540nm and 610 nm, the respective powers φ_(T) and φ_(T)′ of the entire lenssystem 1 must be made equal to each other. Therefore, from the relationof φ_(T)=φ_(T)′, a first condition formula can be derived as expressedbelow:

φ_(T)−φ_(T)=0  (4).

[0038] The power φ_(T)″ of the entire lens system 1 in the case ofoccurrence of temperature variations is next obtained. When atemperature change Δt takes place, the linear expansion of the lenssystem 1 causes the curvature r to change to r″=(1+αΔt)r. Meanwhile, thediffracting surface has a phase function φ(r_(a)) which is expressed asfollows:${\varphi \left( r_{a} \right)} = {{\frac{\Pi \quad r_{a}^{2}}{\lambda_{0}}\varphi_{a}} - {2n\quad {\Pi.}}}$

[0039] Therefore, powers φ_(a)″ and φ_(b)″ of the diffracting surfacesobtained respectively on the first surface 2 and the second surface 3when the temperature change Δt takes place become respectively asexpressed below:${\varphi_{a}^{''} = {\frac{1}{\left( {1 + {{\alpha\Delta}\quad t}} \right)^{2}}\varphi_{a}}},{\varphi_{b}^{''} = {\frac{1}{\left( {1 + {{\alpha\Delta}\quad t}} \right)^{2}}{\varphi_{b}.}}}$

[0040] Further, powers φ₁″ and φ₂″ of the refracting optical systembecome respectively as expressed below:${\varphi_{1}^{''} = {\frac{n_{2} - 1}{r_{1}^{''}} = {\frac{n_{2} - 1}{\left( {n_{0} - 1} \right)\left( {1 + {{\alpha\Delta}\quad t}} \right)}\varphi_{1}}}},{\varphi_{2}^{''} = {\frac{1 - n_{2}}{r_{2}^{''}} = {\frac{n_{2} - 1}{\left( {n_{0} - 1} \right)\left( {1 + {{\alpha\Delta}\quad t}} \right)}{\varphi_{2}.}}}}$

[0041] wherein “n₂” represents a refractive index obtained after thechange of temperature and is expressed as follows:$n_{2} = {n_{0} + {\frac{n}{t}\Delta \quad t}}$

[0042] Further, a change in temperature also causes a distance “d”between the first surface 2 and the second surface 3 to change tod″=(1+αΔt)d. As a result, the power φ_(T)″ of the entire lens system 1obtained after the change of temperature becomes as expressed below:$\begin{matrix}{\varphi_{T}^{''} = {{\varphi_{a}^{''} + \varphi_{b}^{''} + \varphi_{1}^{''} + \varphi_{2}^{''} - {\frac{d^{''}}{n_{2}}\left( {{\varphi_{a}^{''}\varphi_{b}^{''}} + {\varphi_{1}^{''}\varphi_{2}^{''}} + {\varphi_{a}^{''}\varphi_{2}^{''}} + {\varphi_{b}^{''}\varphi_{1}^{''}}} \right)}} = \quad {{\frac{1}{\left( {1 + {{\alpha\Delta}\quad t}} \right)^{2}}\left( {\varphi_{a} + \varphi_{b}} \right)} + {\frac{\left( {n_{0} + {\frac{n}{t}\Delta \quad t} - 1} \right)}{\left( {n_{0} - 1} \right)\left( {1 + {{\alpha\Delta}\quad t}} \right)}\left( {\varphi_{1} + \varphi_{2}} \right)} - {\frac{d\left( {1 + {{\alpha\Delta}\quad t}} \right)}{n_{0} + {\frac{n}{t}\Delta \quad t}}{\left\{ {{\frac{1}{\left( {1 + {{\alpha\Delta}\quad t}} \right)^{4}}\varphi_{a}\varphi_{b}} + {\frac{\left( {n_{0} + {\frac{n}{t}\Delta \quad t} - 1} \right)^{2}}{\left( {n_{0} - 1} \right)^{2}\left( {1 + {{\alpha\Delta}\quad t}} \right)^{2}}\varphi_{1}\varphi_{2}} + {\frac{\left( {n_{0} + {\frac{n}{t}\Delta \quad t} - 1} \right)}{\left( {n_{0} - 1} \right)\left( {1 + {{\alpha\Delta}\quad t}} \right)^{3}}\left( {{\varphi_{a}\varphi_{2}} + {\varphi_{b}\varphi_{1}}} \right)}} \right\}.}}}}} & (5)\end{matrix}$

[0043] From the above formula, a second condition formula foreliminating the power fluctuations of the lens system due to temperaturevariations can be derived as expressed below:

φ_(T)″−φ_(T)=0  (6).

[0044] The lens system 1 according to the invention has the above-statedvariables φ_(a), φ_(b), φ₁, φ₂ and “d” arranged to be at such valuesthat satisfy the formulas (2), (4) and (6). The arrangement enables theembodiment to attain an achromatic effect at the wavelengths 540 nm and610 nm with the power of the entire lens system 1 set at “1” so that thelens system 1 can be arranged to have no variation in the entire systempower against variations in temperature. Since there are three conditionformulas for five variables, a solution cannot be uniquely obtained andthus takes various values. One example of the solution indicatesexistence of one arrangement whereby no variation of opticalcharacteristics (power) takes place for variations in temperature. Inthe case of this example, the five variables  _(a), φ_(b), φ₁, φ₂ and“d” are set as follows: φ_(a)=−0.0194024, φ_(b)=0.4925936,  ₁=1.1402412,φ₂=1.3370397 and d=1.4210498. FIG. 2 shows the power variations obtainedin this case in relation to variations in wavelength. As shown in FIG.2, the power varies less than ±0.1% throughout the entire visiblespectrum from 400 nm to 700 nm to indicate that an achromatic effect(achromatism) is adequately attained. FIG. 3 shows the variations ofpower obtained, in the case of the above-stated example, in relation totemperature variations. As apparent from FIG. 3, the power variationsare corrected down to a rate less than 0.005% within a range oftemperature variations ±30 degrees to indicate also an adequateperformance.

[0045] In accordance with the invention, a diffracting optical elementwhich forms the diffracting surface is not required to be in any specialshape. Therefore, a Fresnel-type diffraction grating having a knownsaw-tooth-like grooved shape or a diffraction grating of a binary typehaving a stepped grooved shape can be employed as the diffractingoptical element. In manufacturing the lens system, the diffractingoptical element may be formed either directly on the base material of arefracting lens or formed on a refracting surface after a photosensitivepolymer or the like is thinly applied to the refracting surface.

[0046] While the embodiment is described above by way of example asarranged to have two diffracting surfaces, the invention can be carriedout for a lens system having at least one diffracting surface. In thatcase, the lens system can be more easily manufactured at a lower cost asits structural arrangement becomes simpler to dispense with a processfor optical axis adjustment between the diffracting surfaces. FIG. 4shows a lens system arranged to have such a simpler structuralarrangement as a second embodiment of the invention. In FIG. 4, allelements that are identical to those shown in FIG. 1 are indicated bythe same reference numerals. Referring to FIG. 4, a surface 5 has only arefracting surface and has no diffracting surface there. FIG. 5 showsvariations in power taking place in the second embodiment in relation towavelength variations. FIG. 6 shows power variations taking place in thesecond embodiment in relation to temperature variations. The variablesof the elements of the second embodiment are set as follows:φ_(a)=0.0440682, φ_(b)=0, φ₁=1.4115329, φ₂=1.4928773 and d=1.3396948. Asapparent from FIGS. 5 and 6, although the amounts of variations aregreater than in the first embodiment shown in FIG. 1, the secondembodiment is capable of sufficiently correcting both the chromaticaberration and the power variations due to temperature variations.

[0047] The second embodiment shown in FIG. 4 may be modified to removethe diffracting surface from the first surface 1 and to have adiffracting surface formed on the second surface 5.

[0048] In the case of each of the embodiments described above, theinvention is applied to a single lens. However, the invention is notlimited to single lenses but is of course applicable to a lens system inwhich at least one of diffracting and refracting optical systems iscomposed of a plurality of lenses. In that case, the lens system can beadequately arranged by first obtaining the amounts of power variationsin relation to both wavelength and temperature variations for each ofsurfaces and, after that, by arranging the overall power of the entirelens system to be constant.

[0049] In each of the embodiments described above, the opticalcharacteristics, such as the focal length and the focus position (focalpoint), of the lens system are arranged to be unvarying by adjusting thelens system only. However, since the lens system is supported by a lensbarrel in actuality, the lens system can be arranged to have its overalloptical characteristic unvarying by taking into consideration also theexpansion and the contraction of the lens barrel anticipated to becaused by variations in temperature.

[0050] According to the arrangement of each of the embodiments describedabove, not only the optical characteristics such as a focal length and afocus position can be arranged to be unvarying against temperaturevariations but also an achromatic effect can be attained for a desiredwavelength region. Further, since a plastic lens is usable withoutnecessitating use of any additional mechanism such as a temperaturesensor, the lens system can be manufactured at a low cost. Thearrangement for combining the diffracting optical system and therefracting optical system with each other effectively permitsenhancement in performance and reduction in number of lenses to beincluded.

[0051] In the case of each of the embodiments described above, theoptical characteristics to be compensated for variations are describedto be paraxial powers, i.e., the focal length and the focus position(focal point), of the lens system. In accordance with the invention,however, the optical characteristics to be compensated for variationsare not limited to these characteristics but may include, for example,also a spherical aberration. In cases where the spherical aberration isto be compensated for its variations, an optical system can be arrangedto have its spherical aberration unvarying against wavelength andtemperature variations by deriving, from marginal rays of light otherthan on-axial rays of light, some formulas corresponding to the formulas(2), (4) and (6) disclosed in the foregoing and by obtaining a solutionfrom these formulas. Further, according to the invention, it is alsopossible to include both the spherical aberration and the paraxialpowers (the focal length and the focus position) in the opticalcharacteristics to be compensated.

[0052] The lens system in each of the embodiments described above hasonly the refracting optical system besides the diffracting opticalsystem. However, the invention applies also to a lens system having areflecting optical system such as a plane mirror, a concave mirror orthe like in addition to the above-stated optical systems.

[0053] While a plastic lens is used in each of the embodiments describedabove, the invention is applicable to a lens system composed of amixture of a plastic lens and a glass lens and also to a lens systemcomposed of a glass lens of a large coefficient of thermal expansion.

[0054] Since the lens system in each of the embodiments described abovecan be manufactured at a low cost and yet has a high performance, thelens system is highly suited for the image forming optical system ofoptical apparatuses such as an interchangeable lens, a camera, a copyingapparatus, a projector, etc. Further, a lens system designed on thebasis of the spirit and principle of the invention is applicable to theimage forming optical system of optical apparatuses other than theoptical apparatuses mentioned above, such as a microscope, a binocular,a telescope, a semiconductor-device manufacturing exposure device, anoptical recording/reproducing apparatus, etc.

[0055] According to the arrangement of each of the embodiments describedabove, a lens system which is capable of keeping its opticalcharacteristics substantially unvarying against temperature variationswithin a predetermined range of variations and also attaining anachromatic effect within a predetermined region of wavelengths can bearranged to have a high rate of performance. In accordance with theinvention, therefore, a lens system and an optical apparatus using thelens system can be arranged to excel in performance.

1. A lens system having one or a plurality of diffracting surfaces andone or a plurality of refracting surfaces, in which a power of eachsurface and a distance between the surfaces are set such that an opticalcharacteristic of said lens system remains substantially unvaryingrelative to temperature variations within a predetermined range and thatan achromatic effect is substantially attained for the opticalcharacteristic within a predetermined region of wavelength.
 2. A lenssystem according to claim 1, wherein the optical characteristic includesa focal length (focus position) and/or an amount of sphericalaberration.
 3. A lens system according to claim 1, wherein a power ofeach surface and a distance between the surfaces are set such that theoptical characteristic remain s substantially unvarying relative totemperature variations within the predetermined range, by taking intoconsideration the deformation of at least one surface and a change of atleast one distance between the surfaces caused by the temperaturevariations within the predetermined range.
 4. A lens system according toclaim 1, wherein a power of each surface and a distance between thesurfaces are set such that the optical characteristic remainssubstantially unvarying relative to temperature variations within thepredetermined range, by taking into consideration the deformation of aplurality of surfaces and a change of a plurality of distances betweenthe surfaces caused by the temperature variations within thepredetermined range.
 5. A lens system according to claim 1, wherein apower of each surface and a distance between the surfaces are set suchthat the optical characteristic remains substantially unvarying relativeto temperature variations within the predetermined range, by taking intoconsideration the expansion and contraction of a lens barrel caused bythe temperature variations within the predetermined range.
 6. A lenssystem according to claim 1, wherein said diffracting surface has adiffraction grating which is in a saw-tooth-like sectional shape.
 7. Alens system according to claim 1, wherein said diffracting surface has adiffraction grating which is in a stepped sectional shape.
 8. A lenssystem according to claim 1, wherein said lens system includes a lenshaving said diffracting surface and said refracting surface formed as acommon surface.
 9. A lens system according to claim 8, wherein said lensis a single lens having both said diffracting surface and saidrefracting surface formed on one surface thereof and only saidrefracting surface formed on the other surface thereof.
 10. A lenssystem according to claim 8, wherein said lens is a single lens havingboth said diffracting surface and said refracting surface formed on onesurface thereof and both said diffracting surface and said refractingsurface formed on the other surface thereof.
 11. A lens system accordingto claim 1, wherein said lens system includes a first lens having saiddiffracting surface and a second lens having said refracting surface.12. A lens system according to claim 11, wherein said first lens hasalso said refracting surface besides said diffracting surface.
 13. Alens system according to claim 12, wherein said second lens has alsosaid diffracting surface besides said refracting surface.
 14. A lenssystem according to claim 13, wherein at least one of said first lensand said second lens is a lens having said diffracting surface and saidrefracting surface formed as a common surface.
 15. A lens system havingone or a plurality of diffracting surfaces and one or a plurality ofrefracting surfaces, in which a power of each surface and a distancebetween the surfaces are set such that an optical characteristic of saidlens system remains the same at two different temperatures and that theoptical characteristic remains the same at two different wavelengths.16. A lens system according to claim 15, wherein the opticalcharacteristic of said lens system includes a focal length (focal point)and/or an amount of spherical aberration of said lens system.
 17. A lenssystem according to claim 16, wherein a power of each surface and adistance between the surfaces are set such that the focal length remainsthe same at the two different temperatures, by taking into considerationthe deformation of at least one surface and a change of at least onedistance between the surfaces caused by temperature variations betweenthe two different temperatures.
 18. A lens system according to claim 16,wherein a power of each surface and a distance between the surfaces areset such that the focal length remains the same at the two differenttemperatures, by taking into consideration the deformation of aplurality of surfaces and a change of a plurality of distances betweenthe surfaces caused by temperature variations between the two differenttemperatures.
 19. A lens system according to claim 16, wherein a powerof each surface and a distance between the surfaces are set such thatthe focal length remains the same at the two different temperatures, bytaking into consideration the expansion and contraction of a lens barrelcaused by temperature variations between the two different temperatures.20. A lens system according to claim 15, wherein said diffractingsurface has a diffraction grating which is in a saw-tooth-like sectionalshape.
 21. A lens system according to claim 15, wherein said diffractingsurface has a diffraction grating which is in a stepped sectional shape.22. A lens system according to claim 15, wherein said lens systemincludes a lens having said diffracting surface and said refractingsurface formed as a common surface.
 23. A lens system according to claim22, wherein said lens is a single lens having both said diffractingsurface and said refracting surface formed on one surface thereof andonly said refracting surface formed on the other surface thereof.
 24. Alens system according to claim 22, wherein said lens is a single lenshaving both said diffracting surface and said refracting surface formedon one surface thereof and both said diffracting surface and saidrefracting surface formed on the other surface thereof.
 25. A lenssystem according to claim 15, wherein said lens system includes a firstlens having said diffracting surface and a second lens having saidrefracting surface.
 26. A lens system according to claim 25, whereinsaid first lens has said refracting surface besides said diffractingsurface.
 27. A lens system according to claim 26, wherein said secondlens has said diffracting surface besides said refracting surface.
 28. Alens system according to claim 27, wherein at least one of said firstlens and said second lens is a lens having said diffracting surface andsaid refracting surface formed as a common surface.
 29. A lens systemaccording to claim 1, wherein said lens system includes one or aplurality of plastic lenses.
 30. A lens system according to claim 29,wherein said plastic lens has said diffracting surface.
 31. A lenssystem according to claim 29, wherein said plastic lens has saidrefracting surface.
 32. A lens system according to one of claims 1 to31, wherein said lens system has a positive power as a whole.
 33. Anoptical apparatus comprising a lens system according to claim 32 andmeans for holding a photosensitive body, wherein an image of an objectis formed by said lens system on said photosensitive body.
 34. Anoptical apparatus according to claim 33, wherein said optical apparatusis a camera.
 35. An optical apparatus according to claim 33, whereinsaid optical apparatus is a copying apparatus.
 36. An optical apparatusaccording to claim 33, wherein said optical apparatus is asemiconductor-device-manufacturing exposure device.
 37. An opticalapparatus comp rising a lens system according to claim 32, wherein animage of an object to be observed is projected by said lens system. 38.An optical apparatus according to claim 37, wherein said opticalapparatus is a microscope.
 39. An optical apparatus according to claim37, wherein said optical apparatus is a projector.
 40. An opticalapparatus according to claim 37, wherein said optical apparatus is oneof a binocular and a telescope.
 41. An optical apparatus according toclaim 37, wherein said optical apparatus is an opticalrecording/reproducing apparatus.